Michael A. Hitchman

Interpretation of the temperature dependent g values of the Cu(H2O)2+6 ion in several host lattices using a dynamic vibronic coupling model

The causes of the previously reported temperature dependence of the g values of the Cu(H2O)2+6 ions in Cu2+ doped Zn(H2O)6(GeF6) and the Tutton’s salts M2Zn(H2O)6(SO4)2, where M=K+, Rb+, NH+4, and Cs+, supplemented by new experimental measurements on the K+ salt, have been investigated. The ground state dynamics of the complexes have been modeled on the cubic E×e Jahn–Teller Hamiltonian perturbed by an orthorhombic lattice strain. For each compound, the vibronic energy levels and associated wave functions were calculated numerically, the overall g values at any temperature being given by a thermal average of the g values of the individual vibronic energy levels, because of rapid exchange between the levels. For the Tutton’s salts it was found that the low temperature g values are strongly influenced by the tetragonal component of the lattice strain, with this corresponding to an axial compression of the ligand field. The temperature dependence of the g tensors, on the other hand, was found to depend large...

The Influence of Vibronic Coupling on the Spectroscopic Properties and Stereochemistry of Simple 4- and 6-Coordinate Copper(II) Complexes

Abstract Although vibronic Jahn-Teller interactions almost always cause both 4- and 6-coordinate copper(II) complexes to distort significantly away from regular tetrahedral and octahedral geometries, rather different factors influence the size and nature of the distortions. In the former case, the active mode is a bending vibration and this has a very small force constant which is often influenced by factors such as lattice interactions. The size of the distortion therefore varies widely from one compound to another, sometimes causing the limiting planar geometry to be reached. The very low energy of this bending vibration causes several unusual features to occur in the electronic spectrum of the planar CuCl4 2- ion. In particular, the significant temperature dependence of the band maxima and the vibrational fine structure observed at low temperature both imply that in the excited electronic states the complex has an equilibrium geometry distorted towards a tetrahedron. For 6-coordinate copper(II), the ov...

Effects of vibronic coupling on the EPR spectra of copper(II) doped K2ZnF4

Abstract The single crystal g-values of ≈≈ 1% Cu2+ doped into tetragonal K2ZnF4 measured over a temperature range between 4 and 295 K are reported. The results are interpreted in terms of a predominantly dz2 ground state wavefunction for the CuF4−6 guest species, with a small admixture of dx2-y2 caused by vibronic coupling. To a first approximation both the magnitudes and temperature dependence of the g-values may be described using a model directly analogous to that conventionally used to represent the temperature dependence of the intensity of parity-forbidden electronic transitions. The generality of this model has been investigated by carrying out numerical calculations of the vibronic wavefunctions and energy levels, and values of the ligand-field and warping parameters of the “Mexican-hat” potential surface of the copper(II) guest complex have been derived. The results reported for several other systems where vibronic coupling had been thought to influence EPR parameters are also discussed using the present model.

The Optical-Spectrum of Ba2zn[cu]f6

Abstract The low temperature polarized absorption and magnetic circular dichroism spectra of ≈3% copper (II) doped into the tetragonally compressed sites of Ba 2 ZnF 6 are reported. The interpretation of the optical spectrum requires that the CuF 6 4− octahedra are tetragonally compressed. This implies that the host site strain dominates with respect to the higher order Jahn-Teller coupling. Considerable vibrational fine structure was observed on sharp electronic origins for the spin-orbit components of the 2 A 1g ( z 2 ) → 2 E g ( xz, yz ) transition. It has been found that cubic anisotropy in the orbital reduction parameters is necessary to interpret the spin-orbit splitting of the 2 E g ( xz, yz ) excited state. The magnitude of the splitting has been used to derive the tetragonal components of the ligand field at the equilibrium geometry of the excited state, and this has been compared with values obtained for other similar systems.

Crystal structures and electron paramagnetic resonance spectra of [Cu{P(C5H4N)3}2]Br2·8H2O and Cu2+-doped [Zn{P(C5H4N)3}2]Br2·8H2O, examples of a dynamic Jahn–Teller effect in two dimensions

Crystals of [Cu{P(C5H4N)3}2]Br2·8H2O, studied at 173 K are triclinic, space group P, with unit-cell dimensions a= 9.082(4), b= 11.340(1), c= 9.084(2)A, α= 98.40(4), β= 94.78(3), γ= 98.27(7)° and Z= 1; those of [Zn{P(C5H4N)3}2]Br2·8H2O studied at 293 K are monoclinic, space group C2/m with a= 12.506(6), b= 13.588(7), c= 11.593(7)A, β= 101.62(4)° and Z= 2. The structures were refined to final R= 0.050 for 3192 reflections with I 3.0σ(I), and R= 0.033 for 1737 reflections with I 3.0σ(I), respectively. The centrosymmetric copper complex has two Cu–N bonds [2.002(4)A] considerably shorter than the other four [2.189(5)× 2, 2.191(5)A× 2]. However, the temperature dependence of the EPR spectrum suggests that in fact the complex has a tetragonally elongated octahedral geometry with two possible orientations in the crystal lattice, these differing by interchange of the directions of the long and intermediate Cu–N bond directions. These forms are in dynamic equilibrium, with an activation energy of ≈600 cm–1 for the interchange. Analysis of the relative intensities of the EPR signals observed at ≈ 10 K suggests an energy difference between the structural isomers of ≈4 cm–1. The zinc complex has crystallographic 2/m symmetry with two independent Zn–N distances [2.150(3)× 2; 2.187(3)A× 4]. The temperature dependence of the EPR spectrum of this compound doped with ≈1% Cu2+ is similar to that of the pure copper(II) compound, but with a lower activation energy for interchange of the structural isomers. The dynamic behaviour of the copper(II) complex in the two compounds is discussed in terms of a potential surface obtained by considering the effects of Jahn–Teller coupling and lattice strain interactions. Bonding parameters derived from the electronic spectrum are consistent with the tetragonally elongated octahedral co-ordination geometry proposed.

Metal-ligand bonding parameters and magnetic properties of some previously reported tetragonal nickel(II) complexes

The variation of the magnetic moments of the complexes [Ni(py)4X2](py = pyridine), [Ni(Hpz)4X2](Hpz = pyrazole), [Ni(mim)4X]X (mim = 2-methylimidazole, X = Cl or Br), and [Ni(en)2(NO2)2](en = 1,2-diaminoethane) over the temperature range 300–4.3 K is reported. The magnetic behaviour, which is strongly influenced by the zero-field splitting of the ground state of each complex, has been interpreted in conjunction with the previously reported d—d transition energies in terms of angular overlap and metal–ligand σ- and π-bonding parameters. Though some anomalies are observed, notably the low values of the halide interactions in the pyrazole complexes and the zero-field splittings of the bromo complexes, the parameters which best reproduce the experimental data generally compare favourably with those derived in other studies on similar systems, and with the expectations of simple theory.

EXAFS of a tetragonally compressed Cu(II) compound

EXAFS has been used to show that at room temperature in KCuAlF6, the copper(II) centre is in a distorted tetragonally compressed octahedral environment. The bond lengths determined (1.88 (x2), 2.12 (x4) Angstrom) are in good agreement with the X-ray crystal structure.

Structure, spectroscopic and angular-overlap studies of tris(pyrazol-1-yl)methane complexes

The metal(II) complexes [M{(pz)3CH}2][NO3]2′ where (pz)3CH is the symmetrical tripodal nitrogen-donor ligand tris(pyrazol-1-yl)methane, have been prepared for M = Co, Ni, Cu or Zn, and examined by single-crystal X-ray diffraction and electronic spectroscopy. The cations are centrosymmetric with the copper complex exhibiting a Jahn–Teller distortion. The ligand ‘bite’ angles N–M–N of 83.3(9)–86.2(1)° cause a slight trigonal distortion from octahedral geometry. The tripod ligand produces a relatively strong ligand field, consistent with the rather short metal–nitrogen bond lengths in the complexes. The pyrazole group acts as a moderately strong σ donor and a weak out-of-plane π donor, with the π interaction in the plane of the amine ligand probably being close to zero.

Notes. Electronic spectrum and metal–ligand bonding parameters of the V(H2O)63+ ion

The electronic spectrum of a crystal of NH4V(H2O)6(SO4)2·6H2O is reported and the transition energies interpreted in conjunction with previously reported magnetic susceptibility data using the angular overlap model. Satisfactory agreement with experiment can be obtained only if the π bonding in the plane of each water molecule is significantly weaker than that perpendicular to this plane.

Fine structure due to magnetic dipole coupling in the low temperature EPR spectrum of Rb2Cu(SO4)2⋅6H2O

A study of the EPR spectrum of single crystals of Rb2Cu(SO4)2⋅6H2O over the temperature range 290 to 115 K is reported. Below ∼240 K unusual fine structure occurs for certain orientations of the magnetic field H, consisting of eight equally spaced lines when H is approximately along the z molecular axis and five equally spaced lines when H lies along the y molecular axis of the Cu(H2O)62+ ion. The fine structure may be explained largely in terms of a magnetic dipole coupling with four neighboring copper (II) ions, each of which produces an approximately equal splitting of the EPR lines at the central copper (II) ion; the five line pattern occurs when the hyperfine interaction with the copper nucleus is small, while the eight line pattern results from a hyperfine interaction which is essentially equal to the magnetic dipole interactions (these each being ∼120 G). Spectra were simulated by considering the point magnetic‐dipole interactions with the ten nearest Cu(H2O)62+ ions using the expression K=g1g2β2 ...